If a data range for a series does contain empty cells one can think of different
ways to handle those. For example:
a) don't pain't
b) assume zero
c) interpolate value
It should be possible for the user to choose which handling he would like to use.
Further details have to be specified, e.g.
-what about invalid values
-empty values in x-ranges
-zero option in logarithmic charts
-maybe an automatic flag to handle the different default behaviours of scatter
and line charts (Line Chart->don't paint, Scatter->Interpolate).

About placing the UI controls for this:
I think there are basically two places where the controls could be placed.
1. In the chart type dialog (and therefore on first page of the wizard). This is
a property at the diagram, and the chart type is the closed choice. It would
have to appear on all chart-type option panels for the following types: line,
scatter (only if points are connected?), area, net, but also bar/line combi chart.
Due to limited space on that pages I would suggest to use a different place:
2. The options tab page of a data series. It is not ideal, but conforming to the
chart-type specific controls like gap-width, overlap and "bars side-by-side" for
bar and column charts. There is enough space to add radio buttons and maybe
additional check boxes.

->bm, iha: The attached is the patch of this issue, I think it's not the final
patch, I just want to show you what I have done, and to confirm whether it's
correct or not. I will change or improve it according to your suggestions. :-)

->weiz: The things written to the files need to be changed. Please look at the
file format definition:
http://lists.oasis-open.org/archives/office/200702/msg00009.html
The attribute is defined to have the name 'treat-empty-cells' not 'nan-data-handle'.
Furthermore the new token is not of type number but instead three different
string should be written to the file, dependent on the choice:
'use-zero' or 'leave-gap' or 'ignore'. As an example look at the token
'label-position'.
Regarding the UI strings I would call the second radio button 'Use zero' or
'Assume zero' or similar. Thus it is more consistent with the other descriptions.
Please try to move the new controls upwards for line charts. It looks very
strange at the moment with that much unused space in between. You can change the
positions of the controls also from c++ code. Please try to move them up
depending on the show/hide behavior of the previous controls in
SchOptionTabPage::Reset.
There needs to be done something around the default values also I think. It is
not so nice to have nothing checked when opening the dialog. Have a look how
this is handled for the new label placements
ChartTypeHelper::getSupportedLabelPlacements as an example.

->weiz: Thanks for the patch! I've looked at it and did some additional changes
during your vacation as communicated via mail. When the previous chart feature
CWS chart19 is integrated I will create a new CWS chart22 based on that new
master and check in your patch including my additional changes.

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